Active noise control and active vibration control systems are employed to reduce noise and vibrations induced by internal combustion engines of vehicles. Active noise control systems utilize speakers and microphones to cancel sound emitted from the engine, which has a frequency that is synchronized with the rotational speed of the crankshaft. Active vibration control systems utilize active actuators, such as active engine mounts, to cancel engine induced vibrations, which also have a frequency synchronized with the rotational speed of the crankshaft. Therefore, the effectiveness of an active noise control and active vibration control system depends on an accurate crank angle signal.
Many modern engines have a crankshaft position sensor operable to provide a crank pulse indicating crank angle. The crank pulse usually lacks the resolution sufficient for active noise and vibration control. Therefore, the crank pulse must be processed or conditioned to generate precise crank angle values for use with active noise and vibration control systems.
Some engine manufacturers have developed AFM (Active Fuel Management, formerly called Displacement on Demand) systems to improve the fuel economy of internal combustion engines. An AFM engine operates in a normal mode (all cylinders are turned on) when power above a predetermined threshold is required and in an AFM mode (half of the cylinders are turned off) when power requirement is reduced. To generate the same level of driving torque with a reduced number of active cylinders, AFM mode produces a higher level of firing force, as a result of increased in-cylinder pressures, for each active cylinder. This higher firing force induces higher torque variations, which produce higher level of structural vibrations degrading noise and vibration, or N&V, performance. In addition, the AFM mode firing frequency reduces to half of the normal mode firing frequency, resulting in more excitation to structurally sensitive frequency ranges. Therefore, conventional passive approaches of vibration suppression may not meet the N&V requirement for both AFM mode and normal mode of engine operation. Engine induced N&V issues also arise in engines with high torque pulses including diesel and homogeneous charge compression ignition, or HCCI, engines. One possible solution to suppress the engine induced vibration is to apply active vibration control technology using smart actuators such as active engine mounts.
There are several types of semi-active and active actuators that can be used for engine vibration suppression. An example of a semi-active actuator is a switchable engine mount whose damping characteristic may be electronically switched between soft and stiff by using electro-hydraulic or magneto rheological (MR) technology. With semi-active actuators, the vibration sensitivity may be switched as operating frequency changes, but may not completely cancel the engine vibration. Active actuators, on the other hand, produce force and/or displacement to counteract engine induced vibration. One type of active actuator is the Active Tuned Absorber (ATA), which utilizes inertial force within the actuator. Another type of active actuator is the Active Engine Mount (AEM). The AEM can generate displacement to counteract engine vibration and at the same time support the static load of the engine.